The present invention relates generally to the generation of randomized pulses, and in particular to a random pulse generator having at least three oscillators, wherein the outputs of at least two of the oscillators are combined to disturb the output of a final oscillator.
On-off waveforms with random transition times, i.e., random pulse trains, have found use in a variety of applications. One such class of applications is that of ultra-wide bandwidth radar and other radar-type applications. Random pulse trains can be used to generate electromagnetic impulses at random intervals, thus greatly reducing susceptibility of detection and/or jamming of the radar system by other electronic systems.
Commonly, pseudo-random pulse trains are generated using a linear feedback shift register (LFSR) as the pulse generator. While use of such pulse generators offers some protection from detection and jamming, the resulting pseudo-random pulse trains generally repeat at some regular interval. This periodicity of the pulse train increases the likelihood of detection and jamming of the radar system.
Attempts to improve the randomness of pulse generators have been described. See, e.g., U.S. Pat. No. 4,769,777 issued to Bittle et al. on Sep. 6, 1988 (Bittle et al.). Bittle et al. is fundamentally based on an LFSR, using a phase shift oscillator whose supply voltage is varied in time to interfere with the periodicity of the LFSR by altering the frequency of oscillation of the phase shift oscillator. Devices of Bittle et al. may also use variable thermal feedback to vary the frequency of the oscillator. However, for miniaturized applications, e.g., incorporating a relatively high rate random pulse generator on an integrated circuit device, thermal feedback may not be desirable or practicable for interfering with periodicity. Furthermore, as integrated circuits continue to operate on lower supply voltages, variation of supply voltage may also be impracticable as a means of interfering with periodicity.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate methods of generating random pulse trains.
The above-mentioned problems with random pulse generation and other problems are addressed by the present invention and which will be understood by reading and studying the following specification. Random pulse generators and systems are described herein using at least three oscillators, wherein the output signals of at least two of the oscillators are combined to disturb the output signal of a final oscillator. The final oscillator refers to the oscillator immediately preceding production of the random pulse train. For one configuration, combined output signals of at least two phase shift oscillators are used to modify the feedback signal of a final phase shift oscillator, thus disturbing the output signal of the final oscillator. For another configuration, the output signals of at least two phase shift oscillators are used to drive a subtractor whose output signal is combined with the output signal of a final phase shift oscillator to drive a subsequent subtractor, thus disturbing the output signal of the final oscillator. It is generally preferred that each oscillator have a different period. While pulse generators described herein are capable of exhibiting substantially random behavior and are referred to as random pulse generators, it is recognized that true random behavior is likely unattainable in electronic systems. However, systems designed in accordance with various embodiments of the invention are capable of sufficiently approximating random behavior to find use in applications such as those described above.
For one embodiment, the invention provides a random pulse generator. The random pulse generator includes a first oscillator having an output signal, a second oscillator having an output signal, and a third oscillator having an output signal. The output signal of the first oscillator and the output signal of the second oscillator are combined to disturb the output signal of the third oscillator.
For another embodiment, the invention provides a random pulse generator. The random pulse generator includes a first phase shift oscillator having an output signal and a feedback signal and comprising a first odd number of inverter segments, a second phase shift oscillator having an output signal and a feedback signal and comprising a second odd number of inverter segments, and a third phase shift oscillator having an output signal and a feedback signal and comprising a third odd number of inverter segments. The random pulse generator further includes a filter comprising a resistive component and having a first input of the output signal of the first phase shift oscillator, a second input of the output signal of the second phase shift oscillator, and a third input of the feedback signal of the third phase shift oscillator. The output signal of the first phase shift oscillator and the output signal of the second phase shift oscillator vary a resistance of the resistive component of the filter and the feedback signal of the third phase shift oscillator is coupled to the resistive component of the filter and is modified by varying the resistance of the resistive component of the filter.
For yet another embodiment, the invention provides a random pulse generator. The random pulse generator includes a first phase shift oscillator having an output signal and a feedback signal and comprising a first odd number of inverter segments, a second phase shift oscillator having an output signal and a feedback signal and comprising a second odd number of inverter segments, and a third phase shift oscillator having an output signal and a feedback signal and comprising a third odd number of inverter segments. The random pulse generator further includes a first subtractor coupled to the output signals of the first and second phase shift oscillators, wherein the first subtractor produces a first intermediate potential in response to the output signals of the first and second phase shift oscillators. The random pulse generator further includes a second subtractor coupled to the first intermediate potential and the output signal of the third phase shift oscillator, wherein the second subtractor produces a second intermediate potential in response to the first intermediate potential and the output signal of the third phase shift oscillator.
For a further embodiment, the invention provides a random pulse generator. The random pulse generator includes a plurality of first phase shift oscillators having a combined output signal, a second phase shift oscillator having an output signal and a feedback signal, and a third phase shift oscillator having an output signal and a feedback signal. The random pulse generator further includes a first subtractor coupled to the combined output signal of the plurality of first phase shift oscillators and the output signal of the second phase shift oscillator, wherein the first subtractor produces a first intermediate potential in response to the combined output signal of the plurality of first phase shift oscillators and the output signal of the second phase shift oscillator. The random pulse generator still further includes a second subtractor coupled to the first intermediate potential and the output signal of the third phase shift oscillator, wherein the second subtractor produces a second intermediate potential in response to the first intermediate potential and the output signal of the third phase shift oscillator.
For another embodiment, the invention provides a random pulse generating system. The random pulse generating system includes a random pulse generator having a random pulse train and at least one frequency divider each having an input signal and an output signal. The at least one frequency divider is coupled to the random pulse generator in series. The first frequency divider in the series has the random pulse train as its input signal, and each succeeding frequency divider in the series has the output signal of the preceding frequency divider as its input signal. The random pulse generating system further includes a multiplexor coupled to the random pulse train and the output signals of the at least one frequency divider as parallel inputs, wherein the multiplexor is responsive to a control signal to select one of the parallel inputs as a selected output.
For still another embodiment, the invention provides a method of generating a random pulse train. The method includes producing a first output signal from a first oscillator, producing a second output signal from a second oscillator, and combining the first output signal and the second output signal to disturb an output signal of a third oscillator.
For yet another embodiment, the invention provides a method of generating a random pulse train. The method includes producing a first output signal from a first oscillator, producing a second output signal from a second oscillator, and varying a resistance in response to the first and second output signals. The method further includes modifying a feedback signal of a third oscillator in response to the resistance, thereby disturbing an output signal from the third oscillator, and conditioning the output signal from the third oscillator to substantially remove partial peaks from the output signal from the third oscillator. Substantially removing partial peaks refers to a conditioning of the signal to produce faster and more complete transitions between the rail potentials.
For a still further embodiment, the invention provides a method of generating a random pulse train. The method includes producing a first output signal from a first oscillator, producing a second output signal from a second oscillator and producing a first intermediate potential in response to the first output signal and the second output signal. The method further includes producing a second intermediate potential in response to the output signal of the third oscillator and the first intermediate potential, producing a third intermediate potential in response to the second intermediate potential and a reference voltage, and conditioning the third intermediate potential to substantially remove partial peaks from the third intermediate potential.
The invention further provides methods of pulse generation and pulse generating apparatus of varying scope.